As a semiconductor substrate, Known semiconductor substrates described in Patent documents 1 to 5 below are known. Patent document 1 discloses a semiconductor substrate that is subjected to a surface treatment so as to obtain desired electric characteristics. Specifically, Patent document 1 discloses a p-type compound semiconductor substrate having p-type impurity atoms in the surface thereof, an n-type compound semiconductor substrate having n-type impurity atoms in the surface thereof, a semi-insulating compound semiconductor substrate having Fe atoms or C atoms in the surface thereof, and a compound semiconductor substrate of a first conductivity type having impurities of a second conductivity type in the surface thereof.
Patent document 2 discloses that metal contamination on a GaN substrate surface is set to be no greater than 10×1010 atoms/cm2. Patent document 3 discloses that the number of Si atoms is set to be 3×1013 atoms/cm2, and the number of atoms of an acidic substance is set to be 3×1014 atoms/cm2, on the surface of a group III nitride substrate, in order to achieve good epitaxial growth on the substrate. Patent document 4 discloses that an surface roughness of AlxGayInzN (0<y≦1, x+y+z=1) is set to be 0.15 nm by CMP, to obtain a substrate free of defects and contamination. Patent document 4 also discloses that Al2O3 or SiO2 is used, as an abrasive grain, in the above CMP, and that the pH is adjusted through addition of an oxidizing agent to a polishing solution. Patent document 5 discloses a feature of using a Si-based gas and a Si piece in dry etching of a gallium nitride compound semiconductor.